Delete Node at Beginning in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delete the first node in a singly linked list.

DSA Python

def delete_at_beginning(head):
    if head is None:
        return None
    head = [1]
    return head

Drag options to blanks, or click blank then click option'

Ahead.prev

Bhead.next

Chead

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning head to None removes the entire list.

Using head.prev instead of head.next causes errors.

To delete the first node, we move the head pointer to the next node.

2fill in blank

medium

Complete the code to update the head after deleting the first node in a linked list.

DSA Python

def delete_first_node(head):
    if head is None:
        return None
    temp = head
    head = [1]
    temp.next = None
    return head

Drag options to blanks, or click blank then click option'

Ahead.prev

Btemp.next

Chead.next

Dtemp.prev

Attempts:

3 left

💡 Hint

Common Mistakes

Using head.prev which is not valid in singly linked list.

Not updating head causes the list to remain unchanged.

After deletion, head should point to the next node of the current head.

3fill in blank

hard

Fix the error in the code to properly delete the first node of a singly linked list.

DSA Python

def delete_head_node(head):
    if head is None:
        return None
    head = [1]
    return head

Drag options to blanks, or click blank then click option'

Ahead

Bhead.prev

CNone

Dhead.next

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' attribute which does not exist in singly linked lists.

Not checking if head is None before deletion.

In singly linked lists, nodes have 'next' pointers, not 'prev'. Use head.next to move head forward.

4fill in blank

hard

Fill both blanks to delete the first node and return the updated head.

DSA Python

def remove_first(head):
    if head is None:
        return None
    temp = head
    head = [1]
    temp.[2] = None
    return head

Drag options to blanks, or click blank then click option'

Ahead.next

Bhead.prev

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' in singly linked list.

Not disconnecting the old head's next pointer.

Move head to head.next and disconnect the old head's next pointer.

5fill in blank

hard

Fill all three blanks to delete the first node and return the updated head safely.

DSA Python

def delete_first(head):
    if head is None:
        return None
    temp = head
    head = [1]
    temp.[2] = None
    temp = None
    return [3]

Drag options to blanks, or click blank then click option'

Ahead.next

Bnext

Chead

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Returning old head instead of new head.

Using 'prev' which is invalid in singly linked list.

Update head to head.next, disconnect old head's next, and return new head.

Practice

1. What happens to the head pointer when you delete a node at the beginning of a singly linked list?

easy

A. It points to the last node

B. It becomes null immediately

C. It remains unchanged

D. It moves to the next node in the list

Solution

1. Step 1: Understand the head pointer role

   The head pointer points to the first node in the list.

2. Step 2: Effect of deleting the first node

   Deleting the first node means the head must now point to the second node, which becomes the new first node.

3. Final Answer:

   It moves to the next node in the list -> Option D

4. Quick Check:

   Head points to next node after deletion [OK]

Hint: Head always moves to next node when deleting at start [OK]

Common Mistakes:

• Thinking head becomes null even if list has more nodes
• Assuming head points to last node after deletion
• Not updating head pointer at all

2. Which of the following is the correct Python syntax to delete the first node in a singly linked list with head pointer head?

easy

A. head = head.next

B. head.next = head

C. head = null

D. delete head

Solution

1. Step 1: Understand Python linked list node deletion

   To delete the first node, we move the head pointer to the next node.

2. Step 2: Check syntax correctness

   Assigning head = head.next correctly moves head to the next node. Other options are invalid Python or incorrect logic.

3. Final Answer:

   head = head.next -> Option A

4. Quick Check:

   Assign head to head.next to delete first node [OK]

Hint: Use head = head.next to delete first node [OK]

Common Mistakes:

• Using 'null' instead of 'None' in Python
• Trying to delete node with 'delete' keyword (not valid in Python)
• Assigning head.next = head which breaks list

3. What will be the output linked list after deleting the first node from the list: 1 -> 2 -> 3 -> 4 -> null?

medium

A. 3 -> 4 -> null

B. 1 -> 3 -> 4 -> null

C. 2 -> 3 -> 4 -> null

D. 1 -> 2 -> 3 -> 4 -> null

Solution

1. Step 1: Identify the first node to delete

   The first node has value 1.

2. Step 2: Delete first node and update head

   After deletion, head points to node with value 2, so list becomes 2 -> 3 -> 4 -> null.

3. Final Answer:

   2 -> 3 -> 4 -> null -> Option C

4. Quick Check:

   First node removed, list starts at 2 [OK]

Hint: Remove first node value, start list from second node [OK]

Common Mistakes:

• Removing wrong node (like second node)
• Not updating head pointer
• Assuming list becomes empty

4. Given the code below, what is the error when deleting the first node?

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

head = Node(10)
head.next = Node(20)
head = head.next.next  # Delete first node

medium

A. head becomes None causing loss of list

B. SyntaxError due to invalid assignment

C. No error, list updates correctly

D. RuntimeError due to NoneType access

Solution

1. Step 1: Trace the assignment head = head.next.next

   head.next is node with data 20, head.next.next is None (end of list).

2. Step 2: Effect of assignment

   Assigning head to None loses the entire list, effectively emptying it.

3. Final Answer:

   head becomes None causing loss of list -> Option A

4. Quick Check:

   head = head.next.next sets head to None [OK]

Hint: Assign head = head.next to delete first node, not head.next.next [OK]

Common Mistakes:

• Assigning head to head.next.next instead of head.next
• Expecting list to remain intact after wrong assignment
• Ignoring None assignment effect

5. You have a singly linked list with nodes: 5 -> 10 -> 15 -> 20 -> null. Write the steps to delete the first node and then insert a new node with value 25 at the beginning. What will be the final list?

hard

A. 10 -> 15 -> 20 -> 25 -> null

B. 25 -> 10 -> 15 -> 20 -> null

C. 5 -> 10 -> 15 -> 20 -> 25 -> null

D. 25 -> 5 -> 10 -> 15 -> 20 -> null

Solution

1. Step 1: Delete the first node

   Move head pointer from node 5 to node 10, so list becomes 10 -> 15 -> 20 -> null.

2. Step 2: Insert new node with value 25 at beginning

   Create new node with data 25, set its next to current head (node 10), then update head to new node.

3. Step 3: Final list after insertion

   List becomes 25 -> 10 -> 15 -> 20 -> null.

4. Final Answer:

   25 -> 10 -> 15 -> 20 -> null -> Option B

5. Quick Check:

   Delete first, then add new node at head [OK]

Hint: Delete head by moving to next, then add new node before head [OK]

Common Mistakes:

• Not updating head after deletion
• Appending new node at end instead of beginning
• Forgetting to link new node to old head